The control of particle microstructure is an important commercial activity, useful, for example, in catalysis, electronics, optics, photovoltaics, and energy absorption applications. Control of particle microstructure allows control of physical and electronic properties, and is critical in the development of new functionalized materials. As an example, synthesis of small particle, high surface area inorganic oxides allows good particle dispersion in polymer binder systems for uniform coatings with specific tailored properties, such as light absorption/transmittance, porosity, and durability. It is well known that products having attributes such as small particles, high-surface area, and high porosity (porosity being determined by pore volume and average pore diameter) can be commercially useful in many applications including, without limitation, as catalysts or catalyst supports.
Titanium dioxide is an important material because of its high refractive index and high scattering power for visible light, making it a good pigment in paints and coatings that require a high level of opaqueness. TiO2 is also active as a photocatalyst in the decomposition of organic waste materials because it can strongly absorb ultraviolet light and channel the absorbed energy into oxidation-reduction reactions. If the TiO2 particles are made very small, less than about 100 nm, and if the photoactivity is suppressed by coating the TiO2 particles, transparent films and coatings can be made that offer UV protection. Therefore, TiO2 is a versatile material with many existing, as well as potential, commercial applications.
Several processes have been reported that use titanium tetrachloride, TiCl4, as a starting source of titanium. TiCl4 dissolved in a solvent is neutralized with a base, such as NH4OH or NaOH, to precipitate a titanium-oxide solid that is washed to remove the salt byproducts, such as NH4Cl and NaCl. However, for the reaction between TiCl4 dissolved in a solvent and NH4OH, the inclusion of the salt byproduct, NH4Cl, in the precipitated solid in order to control the physical properties of the titania product has not been known.
U.S. Pat. No. 6,444,189 describes an aqueous process for preparing titanium oxide particles using TiCl4 and ammonium hydroxide followed by filtration and thorough washing of the precipitate to make a powder with a pore volume of 0.1 cc/g and pore size of 100 Å. Inoue et al. (British Ceramic Transactions 1998 Vol, 97 No. 5 p. 222) describe a procedure to make a washed amorphous TiO2 gel by starting with TiCl4 and a stoichiometric excess of NH4OH solution. Publication No. CN 1097400A reacts TiCl4 with NH3 gas in alcohol solution to precipitate NH4Cl salt, but the titanium product is an alkoxide. A hydrated TiO2 is made by removing the NH4Cl and hydrolyzing the separated liquid with water.